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    SubjectRe: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack
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    On Wed, 2018-07-18 at 17:06 -0700, Dave Hansen wrote:
    > >
    > > >
    > > > >
    > > > > -static inline bool can_follow_write_pte(pte_t pte, unsigned
    > > > > int flags)
    > > > > +static inline bool can_follow_write(pte_t pte, unsigned int
    > > > > flags,
    > > > > +     struct vm_area_struct
    > > > > *vma)
    > > > >  {
    > > > > - return pte_write(pte) ||
    > > > > - ((flags & FOLL_FORCE) && (flags & FOLL_COW)
    > > > > && pte_dirty(pte));
    > > > > + if (!is_shstk_mapping(vma->vm_flags)) {
    > > > > + if (pte_write(pte))
    > > > > + return true;
    > > > Let me see if I can say this another way.
    > > >
    > > > The bigger issue is that these patches change the semantics of
    > > > pte_write().  Before these patches, it meant that you *MUST*
    > > > have this
    > > > bit set to write to the page controlled by the PTE.  Now, it
    > > > means: you
    > > > can write if this bit is set *OR* the shadowstack bit
    > > > combination is set.
    > > Here, we only figure out (1) if the page is pointed by a writable
    > > PTE; or
    > > (2) if the page is pointed by a RO PTE (data or SHSTK) and it has
    > > been
    > > copied and it still exists.  We are not trying to
    > > determine if the
    > > SHSTK PTE is writable (we know it is not).
    > Please think about the big picture.  I'm not just talking about this
    > patch, but about every use of pte_write() in the kernel.
    >
    > >
    > > >
    > > > That's the fundamental problem.  We need some code in the kernel
    > > > that
    > > > logically represents the concept of "is this PTE a shadowstack
    > > > PTE or a
    > > > PTE with the write bit set", and we will call that pte_write(),
    > > > or maybe
    > > > pte_writable().
    > > >
    > > > You *have* to somehow rectify this situation.  We can absolutely
    > > > no
    > > > leave pte_write() in its current, ambiguous state where it has
    > > > no real
    > > > meaning or where it is used to mean _both_ things depending on
    > > > context.
    > > True, the processor can always write to a page through a shadow
    > > stack
    > > PTE, but it must do that with a CALL instruction.  Can we define
    > > a 
    > > write operation as: MOV r1, *(r2).  Then we don't have any doubt
    > > on
    > > pte_write() any more.
    > No, we can't just move the target. :)
    >
    > You can define it this way, but then you also need to go to every
    > spot
    > in the kernel that calls pte_write() (and _PAGE_RW in fact) and
    > audit it
    > to ensure it means "mov ..." and not push.

    Which pte_write() do you think is right?

    bool is_shstk_pte(pte) {
    return (_PAGE_RW not set) &&
    (_PAGE_DIRTY_HW set);
    }

    int pte_write_1(pte) {
    return (_PAGE_RW set) && !is_shstk_pte(pte);
    }

    int pte_write_2(pte) {
    return (_PAGE_RW set) || is_shstk_pte(pte);
    }

    Yu-cheng

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